1. Field of the Invention
This invention relates to a projection lens which is used at a magnification in the vicinity of 1/10 times and used at two kinds of focal length, i.e., at two kinds of magnification with the object-image distance being kept constant and in which the F-number is relatively bright and various aberrations are well corrected and which is inexpensive.
2. Description of the Prior Art
In recent years, a method has been used in which a solid state image pickup element is used as the reading sensor in a facsimile apparatus or a digital copying apparatus, and is disposed as a scanning light-receiving element on the image plane to scan an original. Also, as the functions of a facsimile apparatus, not only one-to-one magnification but also the enlarging and reducing functions have been required. Generally, in the case of reduction, a method of thinning out picture elements read by the solid state image pickup element is adopted. In the case of enlargement, however, the method of softly adding such picture elements aggravates the quality of image, and therefore, if the magnification of the projection lens during one-to-one magnification is .beta..sub.1 and the enlargement ratio is .beta..sub.2, projection is effected at a magnification of .beta..sub.1 .times..beta..sub.2 onto the surface of the solid state image pickup element by a projection lens. If, then, an attempt is made to use a fixed focus lens to change the magnification thereof, the fixed focus lens must be moved to vary the projection magnification and also the object-image distance must be changed, and thus, the apparatus becomes complicated and the image performance is deteriorated. Also, if two fixed focus lenses are used with the object-image distance kept constant, the change-over of these lenses becomes complex and the use of the two lenses results in an increased cost.